The U.S. Food and Drug Administration placed clinical holds on two Regenxbio gene-therapy trials after a pediatric participant developed a brain tumor, prompting pause orders weeks ahead of a pending approval decision for one program. The holds affect a Hurler syndrome program in early testing and a Hunter syndrome candidate that had been nearing a regulatory action. Regenxbio confirmed the suspensions and said it is cooperating with regulators. The holds sent Regenxbio shares sharply lower and underscore growing regulatory scrutiny of AAV-based gene therapies following reports of vector integration and oncogenic events. FDA has signaled investigators will examine integration patterns and long-term safety; companies running AAV programs may face additional monitoring requirements and longer data windows for approval. The development could reshape risk assessments for pediatric gene-therapy trials and force sponsors to revisit long-term surveillance plans.